These drawings are for my Winnebago 7-blade trailer socket that’s attached to the rear bumper and include the towed car’s corresponding plug. When I bought the RV, there was a 6 pin socket that the PO had installed after he’d had the original 7-blade removed. He had a trailer brake controller installed up under the dash near the driver so I guess he towed a heavy trailer. And most RV’ers I’ve seen that tow a trailer instead of a tow dolly carry a classic car on it. Well, I don’t have a classic car, and don’t want to tow a regular car on a trailer or on a dolly so I removed the 6-pin and replaced it with the standard 7-blade socket that the RV had come with that the OP had kindly left in a basement compartment.
The replaced socket was attached to the left of the 2″ receiver on the 10,000 lb bumper that came on the RV as an option. The original specs for the RV showed a 5,000 lb bumper was standard. I appreciated that the PO had upgraded it.
The 7-blade socket attaches wires to the various light systems at the back of the RV, with voltage signals as provided by Freightliner. I haven’t found exactly where the connections are made but it’s not that necessary to know. The 7-blade socket ends up with the wires connected to the terminals and supply voltage through the umbilical cable to the car as:
- RT – Right turn lights
- LT – Left turn lights
- Brk – Brake lights
- Run – Running lights, aka Marker lights
- GND – ground
- +12 V – 12 volt supply to keep car’s battery charged when towing
- Bkup – Back up lights; NOT CONNECTED on my setup
One thing that is typical in a standard towing configuration but often not used that I’m using is a proper +12 volt supply. Over the years of reading towing threads on the various RV forums I frequent, I noticed that many would complain about their car’s battery being dead when they arrived at their destination. If that happened enough, it would ruin the battery. So to prevent that, I’d installed a charging circuit. But only had 18 gauge wire on hand when I did so. Did not take long to discover that 18 gauge was too small so there was excessive voltage drop which meant the battery wasn’t being properly charged. So recently I decided to fix that issue with new 10 gauge wiring. Previously while on the road what I did to avoid having the battery go dead was to leave the car’s running lights off. Then soon after getting to my destination, I’d start the car and run it for a couple minutes to top off the battery. If I remembered.
To help avoid a dead battery situation in future, I’ve connected a length of 10 gauge wire to the RV’s house batteries then through a 20 amp auto-resetting circuit breaker. That’s routed a distance of 15 feet to a terminal on the 7-blade socket. The 10 gauge wire has less voltage drop then 18 gauge would have. I want to be able to tow with the side marker lights of the car on because many people on the freeway don’t notice the car being towed so close to the back of an RV and when they want to change lanes they just drift into the lane right behind the RV like the towed car is not there. Especially at dawn & dusk. Having the side marker lights on should prevent that by making the car more noticeable. Before installing the 10 gauge wiring the battery would end up discharged and wouldn’t start the car if I had them on for a day’s trip though so the auxiliary 12 volt supplied should keep the battery in the car topped off, with or without having the marker lights on. I used a 50 amp Schottky diode (found at Amazon) in the car connected to the car’s battery (supplied by the RV’s charging circuit – see schematics below) because it has a lower voltage drop than a standard diode so more voltage and thus charging current makes it to the car’s battery. I connected the ‘charge’ 10 gage wire to the house batteries because there’s higher voltage there when connected to shore power and I sometimes pull into an overnight space and leave the car connected, the chassis battery voltage would be less in that scenario and not charge as well. (I have had one electrical umbilical cable stolen when I left it connected like that so I don’t do it often, but occasionally if the park is remote with few other guest I might). To facilitate charging, I also added a 10 gage ground wire directly from the 7-blade ground terminal to the chassis.
[Update: Jun ’21]
I found that the 10 gauge power wiring did the trick and I’ve not had a problem with a weak battery after any of the many trips I taken towing the car. Even when I leave it connected overnight in freezing weather and I’ve disconnected the umbilical cable to prevent theft, I still have juice enough to start the engine. I no longer worry about it or follow any special procedure, like starting the engine and letting it run for a while. But, I’ve not needed to drive near dawn or dusk and have the running lights on either. I am confident, however, that that would be okay too.
I use a Hopkins coiled cable…to connect the RV to the car. I like the lights in both plugs but don’t use the feature often.
Here’s a graphic of standard wiring. The drawing on the left is how the connector on my RV is wired, looking into the socket. The drawing on the right shows how the Hopkins electrical cable plug is wired, face view. In my case, that cable is a comprised of two plugs, a 7-blade to 6-pin round, which plugs into the 6-pin socket attached to the car as shown in the next graphic.
And below is a graphic of the 6-pin round connector on the car. I’m using that instead of a 7-blade because it’s traditional, it’s of a size that’s convenient to find a place for it on the front of the car, the Hopkins coiled cable designed for this job can be found practically anywhere that sells RV stuff (in case it’s stolen), and it has enough pins for my use that includes all the lighting features I need for towing a Saturn. The graphic on the left shows how the 6-pin socket attached to the car is wired, looking into the socket. The drawing on the right would be when looking into the plug on the cable. Wire colors are typical, but optional.
So, that’s enough background. Here’s my schematics of the RV to Cable to Car wiring:
This next schematic is the one I made up in 2016 just as I was setting things up for towing. I wired it so while towing the RV’s alternator would keep the car’s battery charged…since I wanted to tow with the marker lights on the car turned on. I soon found that the battery in the car would die fairly quickly and I am pretty sure it’s because I only had 18 gage wire with me when I was wiring up the 12 volt supply circuit and the voltage drop was too much to handle the needs of the battery. That’s why I bumped it up to 10 gage this go round.
The schematic also shows a bit more detail of the wiring in the car. I used blocking diodes (made up my own sets, but there are commercial diode packs available) so I could start the car and step on the brakes while connected to the RV without worrying about causing electrical damage. That’s also why I used a 50 amp Schottky blocking diode in the charging circuit. So if the car is started, say to back it up a bit, the car’s alternator won’t feed back to the RV, damaging the charging circuits there.
And here’s a bit more of a professional schematic that incorporates the info shown on the hand drawn schematics above. You can zoom in on this drawing (and anything else on this web page) by pressing and holding CTRL key on your keyboard and scrolling the mouse scroll wheel.
And here’s what the wiring looks like for the L200. Slightly different than the wiring for the SL1 above. Easier too. Either wiring method would be workable for the vast majority of autos.
There are many forms of running/turn signal/brake lights wiring setups that car manufacturers use so it’s always best to be sure of what you have before designing your towing system wiring. For instance, there are many vehicles that combine the turn signals with brake signals so you have to allow for those differences. My setup worked for me and my equipment but readers should take care to understand their own equipment before committing to any certain wiring scheme for towing.